It is known that windspeed can be determined by transmitting a laser pulse into the wind and measuring the Doppler frequency shift of the light backscattered from aerosols and particles moving with the wind. The velocity of the wind v.sub.w in the direction of the laser pulse is: ##EQU1## where f.sub.s is the Doppler frequency shift and .lambda. is the wavelength of the laser pulse. Four proposed windspeed detection systems are discussed by Menzies in "Doppler lidar atmospheric wind sensors: a comparative performance evaluation for global measurement applications from earth orbit"; Applied Optics, Vol. 25, No. 15, Aug. 1, 1986. The principle challenge inherent in these systems is isolating the laser backscatter signal from background noise. These proposed systems utilize either a heterodyne detection system or a Fabry-Perot filter.
The narrowest optical filter bandwidths currently available with significant acceptance angles are obtained with atomic line filters (ALF's) which have acceptance bandwidths on the order of 0.001 nm. Broadband light containing narrowband signal light is passed through a color glass filter which cuts off wavelengths below a threshold value. The signal and remaining noise light enter an atomic vapor that only absorbs the signal light within the atom's 0.001 nm acceptance bandwidth thereby exciting those absorbing atoms to an intermediate energy level. A pump beam further excites these atoms to a second, higher energy level that then decays through various processes including fluorescence, to the ground state of the atom. The emitted fluorescence occurs at wavelengths below the threshold value. A second color glass filter then cuts off any wavelengths above the threshold which effectively permits passage of only the emitted narrowband fluorescence. In effect, the incoming signal has been internally shifted in wavelength to block any background radiation. For measurement of windspeed these filters have two drawbacks, slow response time (about 500 ns for the alkali atoms) and low quantum efficiency which is defined as the ratio of the number of fluorescence photons detected to the number of incoming signal photons.
It is known that an atom excited to a Rydberg level can be made to ionize quickly when subject to an intense electric field. And it is known that a resonance absorption peak can be split into multiple peaks by the application of a magnetic field.